This invention relates to a process for the preparation of conjugated diene-monoalkenyl arene block copolymers having a low polydispersity index and high tensile strength. This invention further relates to the block copolymers prepared by this process.
Block copolymers of conjugated dienes and monoalkenyl arenes are well known in the art and are employed for a variety of purposes, for example, in adhesives and as elastomers. Generally, one block comprises a block derived from a monoalkenyl arene homopolymer or copolymer thereof. Usually, another block comprises an elastomeric polymer, generally derived from a conjugated diene. To obtain triblock copolymers of maximum uniformity, it is desirable to initiate polymerization employing a difunctional lithium initiator.
Anionic polymerization processes are well known in the art, for example, U.S. Pat. No. 4,431,777, incorporated herein by reference. Difunctional initiators for anionic polymerization are well known; see U.S. Pat. Nos. 4,169,115; 4,172,100; 4,172,190; 4,427,837; 4,196,154 and 4,205,016.
Generally, the use of the above-described difunctional lithium-containing initiators results in the preparation of polymerization products having a broader molecular weight range than is desired. In certain applications, particularly adhesives, this may result in inferior product properties. It is recognized that monoalkenyl arene-conjugated diene-monoalkenyl arene triblock polymers of narrower molecular weight distribution tend to produce adhesive formulations having improved shear hold strength and the triblock polymers normally possess improved tensile rupture strength compared to broader molecular weight versions thereof. Accordingly, for many applications, it would be desirable to prepare block copolymers having a relatively narrow molecular weight distribution, especially distributions wherein the ratio of weight average molecular weight (M.sub.w) to number average molecular weight (M.sub.n) is less than or equal to 1.4 and preferably less than 1.3.
One technique that may be employed to produce narrow molecular weight distribution polymers is the addition of certain polar compounds to the reaction mixture. Disadvantageously, however, in the polymerization of butadiene- or isoprene-containing mixtures in the presence of polar compounds, the resulting polymer possesses a high percentage of undesired vinyl functionality, that is, 1,2- or 3,4-addition units. Such addition products are collectively referred to hereinafter as polymers having 3,4-addition microstructure. Polymers having this microstructure generally possess inferior elastomer properties.
In an attempt to narrow the molecular weight distribution, co-initiators, such as lithium alkoxides or N,N,N',N",N"-pentamethyldiethylenetriamine (PMDETA), were used as co-initiators in U.S. Pat. No. 4,960,842. The block copolymers prepared using the triamine co-initiator demonstrated good tensile strength. Pressure sensitive adhesives prepared from such triblock polymers demonstrate good adhesive properties. However, the co-initiators are not incorporated into the polymer backbone of the chains and are left in the solvent recycle stream, which must be removed to acceptable levels in production scale polymerizations. Such removal and disposal in a process stream presents significant problems.
Therefore, there is a need to prepare block copolymers of conjugated dienes and monoalkenyl arenes which have a narrow molecular weight distribution (polydispersity index) and high tensile strength in the absence of co-initiators which remain in the solvent stream.